[单项选择]
设有关系模式SC(Sno, Sname, Sex, Birthday, Cno, Cname, Grade, Tno, Tname)满足函数 依赖集:{Sno→Sname, Sno→Sex, Sno→Birthday, Cno→Cname, (Sno, Cno)→Grade, Tno→Tname}.SC的主码和属性集(Sno, Tno)的闭包分别是()
A. (Sno,Tno)和{Cno,Cname,Grade,Tno,Tname}
B. (Sno, Cno, Tno)和{Sno, Sname, Sex, Birthday, Cno}
C. (Sno, Cno)和{Sno, Sname, Sex, Birthday, Cno, Cname, Grade}
D. (Sno, Cno, Tno)和{Sno, Sname, Sex, Birthday, Tno, Tname}